An overlooked point here is that in this example the "big company" is the platform operator. It is much easier for Facebook or Apple to brush aside a competitor

An overlooked point here is that in this example the "big company" is the platform operator. It is much easier for Facebook or Apple to brush aside a competitor on their own turf than it is for Microsoft to brush aside Dropbox. So I think the question should be phrased as "Should a startup implement an idea on a platform when the platform provider can clone it?"